When Dzhokhar Tsarnaev was taken into custody from the bottom of a boat in the backyard of a Watertown home Friday night, the suspect was bleeding badly and too weak to resist any longer, officials said.

Police believe Tsarnaev he was initially wounded Thursday night in the gunbattle that killed his brother. Police said they found blood in a car he abandoned and blood at a house. Police said he went undetected by the massive manhunt because he had managed to get just one block outside the search perimeter.

It is unclear whether Tsarnaev was hit again during a final volley before his arrest in the boat.

Investigators -- who are expected to be the country's elite counterterror unit -- are hoping that Tsarnaev survives because they are intent on determining what triggered the shocking attack and whether he had any help. The bombing killed three, including a young boy, and wounded about 170. An MIT security officer was allegedly killed by the duo on Wednesday night and a Boston transit cop was badly wounded in a subsequent shootout.

One focus of the probe so far is a six month trip Tamerlan Tsarnaev took to the semi autonomous Russian province of Dagestan in 2012. Dagestan has become a hotbed of militant Islamic activity.

The capture of Dzhokhar Tsarnaev set off a night of celebration in Watertown and Boston, a spontaneous relief after the region was ordered indoors for an entire day as heavily armed SWAT teams searched for the surviving suspect. Jubilant residents high fived police and chanted "Boston strong" and USA.

"It brings a sigh of relief and I think that it really allows us to start the healing process," Boston resident Heather Budda said, according to ABC News Radio. "He's still alive so we still have a chance to hear what the reasoning behind it is."

Crowds gathered around Boylston Street in Boston, the sight of Monday's twin bombing at the finish line of the marathon.

"Let's go Boston," was chanted while others climbed trees and draped themselves in American flags.

The dragnet came to an end shortly after the lockdown was lifted and Watertown homeowner David Henneberry walked into his backyard and saw something amiss with his boat, according to Henneberry's neighbor, George Pizzuto.

"He looked and noticed something was off about his boat, so he got his ladder, and he put his ladder up on the side of the boat and climbed up, and then he saw blood on it, and he thought he saw what was a body laying in the boat," Pizzuto said. "So he got out of the boat fast and called police."

Henneberry notified police, and minutes later gunfire erupted and dozens of law enforcement officers rushed to secure a perimeter around Franklin Street in Watertown, where residents were immediately warned to stay indoors and "shelter in place."

According to police, a helicopter with infrared technology then located Tsarnaev in the boat and noted that he was moving about within it. The helicopter directed officers on the ground to the boat, where they briefly exchanged gunfire shortly before 7 p.m.